Hundreds of supporters of slain Russian opposition leader Boris Nemtsov gathered Tuesday at the site near the Kremlin where he was killed to commemorate his death. 

Friends and ex-colleagues of the murdered Kremlin critic held a "Minute of no silence" 40 days after he was gunned down on a central Moscow bridge.

Two bomb attacks early Tuesday shook Ukraine's second city Kharkiv, near the Russian border and the frontline of the country's war, causing some damage but no casualties.

The Kharkiv prosecutor's office said a probe was underway for a "terrorist attack" after a strong explosion in the city center, which knocked out windows in a nearby university building and setting car alarms screeching.

The young Cossack fighter gripped his automatic rifle as he swore he would defend the town of Perevalsk in war-torn east Ukraine that he and his comrades now control. 

"We're expecting an attack. We're prepared, no one can enter the town," the man in his twenties said, a traditional Cossack fur hat on his head.

Russia's Foreign Minister Sergei Lavrov said Monday he was "dismayed" that a Saudi-led coalition did not consult the U.N. Security Council prior to carrying out strikes in Yemen and urged negotiations to resolve the crisis.

"Right now (the operation) does not have any foundation in international law. Of course we were a little dismayed, to put it mildly, that the operation was begun without any consultations," he said in an interview with RIA-Novosti news agency.

Czech President Milos Zeman and the U.S. ambassador to Prague have clashed over the former's plan to travel to Moscow for the World War II victory anniversary, an event largely snubbed by western leaders.

In a TV interview last week, U.S. ambassador Andrew Schapiro questioned the staunchly pro-Russian Zeman's wish to be "probably the only EU head of state" to attend the May 9 showcase parade on Moscow's Red Square.

Talks on ending the war in Syria began on Monday in Moscow but the absence of key opposition groups meant there was little hope of progress in resolving the conflict.

Syria's ambassador to the United Nations, Bashar al-Jaafari, is heading the government delegation for the meetings with members of the domestic "tolerated" opposition, the National Coordination Committee for Democratic Change (NCCDC).

Six Ukrainian troops were killed in two separate incidents in the troubled separatist-held east Sunday as isolated clashes continue to violate a fragile ceasefire to end the year-long war, officials said.

"Unfortunately, six Ukrainian soldiers have been killed today," army spokesman Oleksandr Motuzyanyk told AFP.

A year since the start of the fighting in eastern Ukraine, Vladimir Putin may not have emerged the winner in his showdown with the West but he has not lost either, analysts say.

By supporting Ukrainian separatists, they say, he took a huge risk but it largely paid off as it allowed him to punish Kiev's pro-Western authorities for seeking to turn their back on Russia and stand up to the West.

A key activist in Syria's "tolerated" opposition said Saturday he cannot attend peace talks in Moscow next week because Damascus refuses to lift his travel ban.

"Neither I nor my movement will take part in the meeting in Moscow because the regime refuses to lift my travel ban," Louay Hussein told AFP.

Three soldiers died when a mine exploded in Ukraine's separatist-held east where isolated clashes continue to undermine a ceasefire aimed at ending a year-long war.

Army spokesman Andriy Lysenko said Saturday that "as a result of the explosion of an enemy mine ... three Ukrainian soldiers were killed, two injured" in the last 24 hours.